This invention relates to closable bags and envelopes and, more particularly, it concerns an improved closure sealing arrangement for envelopes of the type in which a narrow band of pressure sensitive adhesive, initially covered by a releasable liner strip, is effective to seal opposing panel surfaces at the opening through which the envelope is filled.
Envelopes formed from pliable thermoplastic sheet materials have become increasingly popular for use by courier service organizations to package documents and other items to be shipped usually on an expedited basis. Such envelopes are especially suited to this use because of the relatively high strength of the plastic sheet material, resistance to damage by water and other liquids, imperviousness to moisture generally and because of the ability of the sheet material to be printed in a wide range of distinguishing colors and styles. Additionally, the plastic sheet materials of such envelopes are receptive to a variety of pressure-sensitive adhesives by which a strong and secure sealed closure of the envelope may be effected after it is filled.
Typically, plastic courier envelopes are formed by transversely severing and heat sealing overlying portions of a continuous longitudinally folded plastic web. In this way, the bottom of the envelope is formed by a fold in the continuous web whereas side edges of the envelope are defined by heat fusion lines securing front and back panels of the envelope. A closure flap is typically provided by folding the web on a line displaced from the transverse center thereof and so that the open top end of the envelope rear panel projects past the top edge of the front envelope panel as a rectangular flap to be folded back on and adhered to the front face of the envelope front panel. That portion of the web resulting in the closure flap is provided with a continuous strip of pressure sensitive adhesive covered by a continuous liner strip of material which releases easily from the adhesive. The liner strip is severed from the web with the remainder of the envelope during severance along the heat fusion lines aforementioned.
A major problem associated with flap closures of the type described in the environment of courier envelopes is that the adhesive strip is spaced substantially from the top edge of the envelope front panel. Thus, while the flap may be securely attached in its closed condition over the front envelope panel, no sealing occurs along the edges of the flap with the result that substantial openings are left at the top corners of the closed envelope. In addition, the pliable nature of the plastic material from which the envelope is formed requires the person who closes the flap to secure the strip of pressure sensitive adhesive in a final closed position over the front wall of the envelope. In so doing, the material offers no assistance to the operator in reaching the closed position of the flap as would occur, for example, in a paper envelope or an envelope made of relatively rigid material where a fold line tends to guide the flap into its closed position. Accordingly, there is a need for an improvements in closure seal arrangements for envelopes of the type referred to.